Does IPhone 4 use Wi-Fi first or cellular?

I just got an iPhone 4 and I don't want to waste my data so I want to put Wi-Fi and cellular at the same time but I don't know if Wi-Fi goes first or cellular

With an iPhone 4, if you connect it to the Wi-Fi it wil start automatically connecting when you're at the place where you connected it in the first place. If you disconnect from it or if you get to far away from the Wi-Fi spot it will automatically connect to 3G

If you are in range of a known Wi-Fi network, it will use Wi-Fi. If not, it will use celluar data.